There are two ways of winding a long length of film: one is winding it onto a core (core winding) and the other is winding it without a core (coreless winding). The core winding method is not suited to automatic and high-speed processing of an exposure film, because at the beginning of winding the leading end of the film should be securely fixed to the core and an appropriate handling of the core is necessary in the developing process.
In a coreless winding method, a film is continuously wound (or involuted) in a cylindrical container. The problem with this method is that friction occurs between the layers of the roll of film. When film is continuously wound in a container of a fixed diameter, there must be some slip between the adjacent layers of the film, as well as between the film and the inside wall of the container. The area of the slippage increases as the film is wound in the container, and the frictional force increases as the slipping area increases. When the length of the film wound in the container exceeds a certain amount, the frictional force becomes too large, and the film can no longer be fed into the container and the film begins to jam at the entrance of the container.
A mechanism for alleviating this problem is shown in the Japanese Unexamined Patent Application No. H2-39145 in which the radius of the roll of film wound is allowed to increase as the film is wound. The mechanism is constructed as follows. In a container, two driving rollers are disposed at the bottom, two rollers are disposed at the side and three rollers are provided at the top to form an inscribed cylinder in which a film is wound. The two side rollers are held by an arm and the three top rollers are held by another arm, where both arms can swing in the container and are urged by respective springs towards the inscribed cylinder. As more film is wound in the inscribed cylinder, the wound film tends to evolve and exerts an expanding force on the cylinder by virtue of its elasticity. The arms (and the rollers held by the arms) thus swing outward, allowing increase in the outer radius of the roll of the wound film.
Since the outer radius of the film increases as the film is wound, the newly wound portion of the film is wound on the pre-existing roll of the already wound film. This means that no slip in the roll of the already wound long film occurs. Thus, using the prior art winding mechanism, a long film can be smoothly fed into the container with almost a constant force.
The prior art winding mechanism, however, needs many rollers to define a well-shaped inscribed cylinder for smoothly winding the film, and a complicated swing mechanism is also required for each arm in order to increase the radius of the inscribed cylinder while keeping an adequate cylinder shape.